1. Field of the Invention
This invention relates to abrasive tools, and more particularly to grinding wheels having a single layer of abrasive bonded to a textured cutting face.
2. Background Information
Single layer metal bonded abrasives are used to form the cutting surfaces of various cutting tools such as core drill bits, diamond saw blades and single abrasive layer grinding wheels. These cutting tools are useful for cutting and abrading relatively hard materials such as metal, concrete, stone, ceramics and the like, as well as for drilling subterranean formations in oil and gas recovery. Such cutting tools are normally constructed from a core or blade support material such as steel or aluminum and a superabrasive such as diamond or cubic boron nitride (CBN) bonded to a cutting face of the core.
While effective in many of these applications, single layer bonded abrasive tools have not been particularly effective in some relatively difficult, precision grinding operations. An example of such a precision grinding application includes aerospace creepfeed grinding where thermal damage to the workpiece is problematic. Another example is bi-metallic engine block deck face grinding, in which blocks consisting of dissimilar metals such as cast iron with aluminum inserts, must be ground to precision tolerances. In this application, burr formation at the interface of the dissimilar metals is particularly troublesome. In both of these applications, it is difficult to apply coolant to, and remove the grinding swarf from, the grinding zone or point of contact between the wheel and workpiece.
One attempt to prevent chips from clogging the abrasive grain is disclosed in European patent application No. EP 0770457 A1. This reference discloses a grinding wheel having a number of pyramidal or truncated-pyramidal projections formed on a portion of a metal base. Super abrasive grains having grain sizes which are smaller than the heights of the projections are fixed to the surfaces of the projections. A coating film consisting of fluororesin is formed to at least partially cover the outer surface of the grinding face including the surfaces of the super abrasive grains, for "preventing deposition of a workpiece." The grains are bonded to the grinding face by electrodeposition or electroplating. While this construction may provide advantages in terms of chip removal in some applications, the use of the fluororesin layer adds complexity and cost to the manufacture of the grinding wheel. Moreover, electrodeposited bonds, although historically considered to be optimal for heavy duty use, have generally proven undesirable for use wheels adapted for precision grinding, particularly those which employ textured cutting faces. In part, this is because the bond has insufficient strength to resist the pressures of such applications, so that the abrasive grain and bond tend to break free or peel from the cutting face prematurely. This breaking or peeling tends to reduce tool life while the loose abrasive also tends to score the workpiece, thus degrading the quality of the surface finish. This phenomenon is particularly problematic in textured cutting faces as grinding contact is made with a relatively small number of grains (those on the apex of the projections), which accordingly experience relatively high grinding forces per unit area of contact.
One explanation for this relatively weak bond is that electrodeposited bonds serve only to mechanically entrap the individual abrasive grains and do not form a chemical bond with the grain.
Another disadvantage of this approach is that electrodeposition tends to attenuate the texture of a cutting face by permitting the grain to gather or collect in the recesses between projections. An example of this attenuation or collection of grains is shown in EP 0393540B1. One approach to address this problem would be to provide a greater degree of texture to the core supporting the abrasive, such as by milling a series of relatively deep grooves in the cutting face, to compensate for the attenuation. However, conventional milling operations tend to be time consuming and thus relatively expensive.
Brazed bonds are an alternative to electrodeposition and offer the potential advantage of improved bond strength. Historically, however, due to manufacturing concerns, brazed bonds have been selected less frequently than electroplated bonds for use in single layer superabrasive tools. Tools made using soft brazes have been typically directed to less demanding non-abrasive tool applications. Use of harder brazes has been discouraged because diamond and CBN abrasives tend to thermally degrade due to oxidation at the higher melting temperatures associated with these brazes. Moreover, the harder bonds provided by some brazed bonds such as molybdenum/iron alloys disclosed in U.S. Pat. No. 3,894,673 tend to have a significantly different coefficient of thermal expansion than the diamond abrasive, which introduces certain stresses to the diamond crystals which are not relieved to the same extent as in softer, lower melting point brazes, thus tending to reduce tool life.
One example of a highly textured or contoured tool which utilizes a braze bond has been disclosed in commonly assigned International Publication No. WO 97/33714. This disclosure, however, is directed to cutting tools, namely saw blades, rather than to grinding wheels, and utilizes a substrate having a plurality of relatively large teeth coated with abrasive grain. These teeth-like geometric shapes must be milled into the core prior to brazing the abrasive onto the core, necessitating an expensive milling step, or other similar manufacturing step, to produce a profiled core for the tool.
A need thus exists for an improved grinding wheel having a single layer of abrasive grain and a textured cutting surface manufactured without expensive or difficult processes, which is adapted for use in heavy duty precision grinding applications.